Method of deionizing solution yielded by polyester decomposition with ethylene glycol

ABSTRACT

There is provided a method for deionizing a decomposition produced solution resulting from decomposition of a polyester by ethylene glycol. The ester interchange reaction and hydrolysis reaction along with cation removing treatment of a decomposition product resulting from decomposition of a polyester by ethylene glycol are suppressed. Thereby, a method for deionizing the decomposition produced solution with small reductions in yield and purity can be provided.

TECHNICAL FIELD

The present invention relates to a method for deionizing a decompositionproduced solution resulting from decomposition of a polyester byethylene glycol. More specifically, the present invention relates to amethod for removing ions as impurities from a decomposition producedsolution resulting from decomposition of a polyester having a highcontent of ions as impurities by ethylene glycol, particularly adecomposition produced solution containing, as a main component,bis(2-hydroxyethyl)terephthalate resulting from decomposition of arecovered polyethylene terephthalate by ethylene glycol.

BACKGROUND ART

A polyester, particularly polyethylene terephthalate, is widely used infields of various molded articles such as a fiber, a film and a resin.The polyethylene terephthalate is generally produced by a method whichcomprises reacting dimethyl terephthalate or terephthalic acid withethylene glycol in the presence of a catalyst. At that time, to enablethe polyethylene terephthalate to satisfy properties required for aspecific application, functional additives such as a stabilizer, acolorant and an antistatic agent are used.

In recent years, disposal of a polyethylene terephthalate moldedarticle, particularly a post-consumer polyethylene terephthalate bottle(PET bottle) is now a social problem in that it pollutes theenvironment. Accordingly, its collection and recycling are under way.

As one method therefor, a method comprising the steps of collecting aused polyethylene terephthalate molded article, crushing the articleinto chips or flakes, depolymerizing them with ethylene glycol,purifying the resulting product so as to obtain high-puritybis(2-hydroxyethyl)terephthalate, and then polymerizing thebis(2-hydroxyethyl)terephthalate so as to obtain polyethyleneterephthalate is under study.

Based on an idea that in order to obtain a polyester of high quality, araw material of the polyester must also be of high quality, the presentinventor has studied the method comprising the steps of depolymerizingchips or flakes of collected polyethylene terephthalate with ethyleneglycol and purifying the resulting product so as to obtain high-puritybis(2-hydroxyethyl)terephthalate. Then, as a finding obtained from thestudy, the present inventor has proposed a method in which abis(2-hydroxyethyl)terephthalate solution composition containingethylene glycol, bis(2-hydroxyethyl)terephthalate and cations and anionsas impurities as a solution resulting from the depolymerization reactionis brought into contact with a cation exchanger and an anion exchangerso as to give high-purity bis(2-hydroxyethyl)terephthalate (for example,refer to patent document 1).

Further, as a result of further study on the method, the presentinventor has found that since hydrogen ions are liberated by theforegoing cation exchange treatment, a resulting treated solutionbecomes strongly acidic and bis(2-hydroxyethyl)terephthalate thereforebecomes liable to cause an ester interchange reaction with diethyleneglycol or a hydrolysis reaction, so that a reduction in yield of thetarget bis(2-hydroxyethyl)terephthalate or a reduction in purity of thebis(2-hydroxyethyl)terephthalate caused by inclusion of the abovereaction products in the bis(2-hydroxyethyl)terephthalate are liable tooccur disadvantageously.

(Patent Document 1)

International Publication No. 01/10812 Brochure

An object of the present invention is to provide a method for deionizinga decomposition produced solution resulting from decomposition of apolyester by ethylene glycol, particularly a decomposition producedsolution containing, as a main component,bis(2-hydroxyethyl)terephthalate resulting from decomposition of arecovered polyethylene terephthalate by ethylene glycol.

Another object of the present invention is to provide a deionizationmethod in which an ester interchange reaction and hydrolysis reaction ofa decomposition product resulting from decomposition of a polyester byethylene glycol, i.e., bis(2-hydroxyethyl)terephthalate, along withcation removing treatment are suppressed.

Still another object of the present invention is to provide adeionization method in which a reduction in yield and a reduction inpurity which are caused by an ester interchange reaction or hydrolysisreaction caused by a decomposition product resulting from decompositionof a polyester by ethylene glycol hardly occur.

Other objects and advantages of the present invention will be apparentfrom the following description.

DISCLOSURE OF THE INVENTION

According to the present invention, the above objects and advantages ofthe present invention are achieved by a method for deionizing adecomposition produced solution resulting from decomposition of apolyester by ethylene glycol, the method comprising causing the solutioncontaining a decomposition product, ethylene glycol and cations andanions as impurities to contact with a cation exchanger at a temperatureof at most 100° C. for a residence time of 5 to 60 minutes and then withan anion exchanger within 30 minutes immediately thereafter so as toreduce contents of the cations and anions.

A starting material used in the present invention is a decompositionproduced solution resulting from decomposition of a polyester byethylene glycol. The solution contains a decomposition product, ethyleneglycol, and cations and anions as impurities. The solution is obtainedby decomposing a polyester by an excessive amount of ethylene glycol.

The polyester used in the present invention may be produced by anymethod. Illustrative examples of the polyester include homopolymers suchas a polyethylene terephthalate and polyethylene naphthalate, andcopolymers thereof such as copolyesters resulting from copolymerizationwith isophthalic acid or 1,4-cyclohexane dimethanol and copolyestersresulting from copolymerization with 1,4-butanediol. Of these, thepolyethylene terephthalate and copolymers thereof are particularlypreferred. For a polyester produced by a polycondensation reaction,glycolysis is generally easy and suitable for the present invention. Asolvent used in the glycolysis may contain glycols other than ethyleneglycol.

Further, the cations and anions as impurities in the present inventionare derived from catalysts for glycolysis (for example, alkali compoundssuch as sodium hydroxide and potassium hydroxide), catalysts forpolymerization of a polyester (for example, antimony compounds such asantimony oxide, germanium compounds such as germanium oxide, andtitanium compounds such as titanium alkoxide), and additives such as astabilizer (such as a phosphorus compound) and an antistatic agent.However, ions originated from a variety of contaminations which are noteasily predicted and stuck to or accompanied with the polyester are alsoincluded.

The foregoing decomposition produced solution (hereinafter may bereferred to as “solution compositions”) is brought into contact with acation exchanger and an anion exchanger. The solution composition can bedeionized by, for example, causing the solution composition to passthrough layers of the ion exchangers filled in a column or the like soas to make them contact with each other. When the solution compositionis a suspension, clogging occurs in the ion exchanger layers, andpartial flow are caused by insufficient permeation of the solutioncomposition or permeation resisting spots, so that stable deionizationtreatment becomes difficult to achieve. Therefore, after solidimpurities such as fine particles each having a size of not smaller 1 μmare removed from the solution composition as required, the solutioncomposition must be brought into contact with the cation exchanger andthe anion exchanger with the temperature of the solution compositionbeing a temperature which is equal to or lower than maximum usabletemperatures of the ion exchange resins and at which crystals ofethylene glycol ester of dicarboxylic acid, particularlybis(2-hydroxyethyl)terephthalate, are not precipitated from a glycolysisreaction solution. It is preferred that the solid impurities having amaximum diameter of not smaller than 1 μm are removed. The removal ofsolid impurities can be done by a percolation using a diatomite, a fiberfilter etc.

In general, the maximum usable temperature of the cation exchanger ishigher than that of the anion exchanger. Accordingly, the solution to betreated may be cooled to a temperature equal to or lower than themaximum usable temperature of the anion exchanger after cation exchangetreatment or may be subjected to the cation and anion exchangetreatments at a temperature equal to or lower than the maximum usabletemperature of the anion exchanger. Since the proportion of cations inion impurities is generally predominantly larger than that of anions, itis preferable to carry out the anion exchange treatment after the cationexchange treatment.

The treated solution becomes acidic due to hydrogen ions generated bythe cation exchange treatment. As a result, a decomposition productresulting from decomposition of a polyester by ethylene glycol is liableto cause an ester interchange reaction with coexisting diethylene glycolor a hydrolysis reaction with contained water. Under the circumstances,the present inventor has made intensive studies so as to provide themethod for deionizing the decomposition produced solution (solutioncomposition) in which the ester interchange reaction and the hydrolysisreaction are suppressed so as to achieve small reductions in yield andpurity. As a result, the present inventor has completed the presentinvention.

Preferably, firstly, in the present invention, a hydrolysis reaction ofa decomposition product resulting from decomposition of a polyester byethylene glycol is suppressed by reducing the content of water in aglycolysis reaction solution. In general, glycolysis is carried out at atemperature higher than or equal to the boiling point of glycol.Accordingly, when a distillation column is disposed in a glycolysisreactor so as to distill out the water from the reaction solution, theamount of the water in the reaction solution can be reduced so as tosuppress the hydrolysis reaction. Meanwhile, evaporated glycol may beput back to the glycolysis reactor.

Preferably, secondly, in the present invention, an ester interchangereaction of the decomposition product resulting from decomposition ofthe polyester by ethylene glycol is suppressed by shortening residencetime of cation exchange treatment. As the temperature at which the ionexchange treatment is carried out increases, a rate of ion exchange alsoincreases. However, since a rate of the ester interchange reaction ofthe decomposition product also increases along with the above increases,the residence time is shortened so that an amount of the decompositionproduct converted by the ester interchange reaction does not exceed anacceptable value.

Preferably, thirdly, in the present invention, the ester interchangereaction and hydrolysis reaction of the decomposition product resultingfrom decomposition of the polyester by ethylene glycol are suppressed bycarrying out anion exchange treatment as soon as possible aftercompletion of the cation exchange treatment. Since hydroxide ions aregenerated by the anion exchange treatment and cause a neutralizationreaction with hydrogen ions, hydrogen ions in the reaction solution canbe decreased.

Best Mode for Carrying Out the Invention

Hereinafter, the present invention will be described in detail withreference to specific examples. A polyethylene terephthalate is used asthe polyester, and glycolysis is carried out by use of ethylene glycol.The temperature of the glycolysis is preferably 160 to 300° C., morepreferably 180 to 240° C. The weight ratio between the polyethyleneterephthalate and ethylene glycol is preferably 1:9 to 3:7. When theamount of the polyethylene terephthalate is too small with respect tothat of ethylene glycol, the amount of bis(2-hydroxyethyl)terephthalateto be produced becomes smaller than saturation solubility to ethyleneglycol, so that bis(2-hydroxyethyl)terephthalate can be obtained only inan amount smaller than a maximum yield obtained with respect to theamount of whole solution to be subjected to the deionization treatments,which is not economical. On the other hand, when the amount of thepolyethylene terephthalate is too large with respect to that of ethyleneglycol, an oligomer of bis(2-hydroxyethyl)terephthalate increases, sothat the yield of bis(2-hydroxyethyl)terephthalate decreases. Further,when bis(2-hydroxyethyl)terephthalate exists beyond the saturationsolubility to ethylene glycol, bis(2-hydroxyethyl)terephthalate isprecipitated, so that the deionization treatments cannot be carried out.

The glycolysis can be carried out by a conventionally known method.Illustrative examples of such a method include a method comprisingmixing a molten polyester with ethylene glycol,bis(2-hydroxyethyl)terephthalate or a mixture thereof and depolymerizingthe mixture, a method comprising the steps of mixing a molten polyesterwith ethylene glycol, bis(2-hydroxyethyl)terephthalate, a low degreepolymer composition (oligomer) comprisingbis(2-hydroxyethyl)terephthalate as a recurring unit, or a mixturethereof so as to pre-depolymerize the mixture and then mixing productsresulting from the pre-depolymerization with ethylene glycol so as todepolymerize the mixture, and a method comprising the steps of charginga pulverized polyester and ethylene glycol into a glycolysis reactor andcausing them to react with each other. The method using a glycolysisreactor is preferably carried out with a distillation column placed inthe glycolysis reactor, while water is removed from a reaction solutionto out of the system by distillation. In this case, it is preferable toput evaporated glycol back into the system. By carrying out theglycolysis in such a manner, the amount of water in the solutioncomposition to be brought into contact with a cation exchanger can bereduced, so that a hydrolysis reaction which occurs along with cationremoving treatment can be thereby suppressed. It is preferable that theamount of the water contained in the solution composition to be broughtinto contact with a cation exchanger be adjusted to an amount of nothigher than 0.5 wt %.

The reaction solution obtained by the glycolysis containsbis(2-hydroxyethyl)terephthalate as a main component as well as anoligomer of bis(2-hydroxyethyl)terephthalate, diethylene glycolcontained in the raw material polyethylene terephthalate, diethyleneglycol and 2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl]terephthalateproduced from ethylene glycol, and the like. Further, the reactionsolution also contains impurity ions derived from a catalyst used in theglycolysis (such as sodium hydroxide), a catalyst used in apolycondensation reaction of the polyethylene terephthalate (such asgermanium oxide, antimony oxide, manganese acetate or titaniumalkoxide), a stabilizer such as a phosphorus compound, a colorant, and avariety of contaminations which are not easily predicted.

To remove the above impurities from the glycolysis reaction solution,the glycolysis reaction solution is subjected to adsorption treatmentusing activated carbon so as to remove the colorant and impuritiesadsorbable to activated carbon and then brought into contact with acation exchanger and an anion exchanger or the cation exchanger and amixed layer of the cation exchanger and the anion exchanger. The cationexchanger and the anion exchanger may be particles, chains, fibers oramorphous, for example. When they are in the form of particles, they canbe brought into contact with the glycolysis reaction solution by, forexample, filling them in a column and causing the glycolysis reactionsolution to flow through the column.

As the cation exchanger, a cation exchange resin is preferred, and asthe anion exchanger, an anion exchange resin is preferred. Illustrativeexamples of cation exchange functional groups in the cation exchangeresin include —SO₃H and —COOH. Further, as the cation exchange resin,commercial products such as DIAION SK1B, SK104, SK110, SK112 and SK116(products of Mitsubishi Chemical Corporation) and AMBERLITE IR120B Na,IR120BN Na, IR124 Na and 200CT Na (products of Rohm & Haas Co., Ltd.)can be used. In these commercial products, ion exchange functionalgroups are generally stabilized as salts such as sodium salts, and theyare generally converted into free acid radicals such as those describedabove upon use of the products.

Illustrative examples of the anion exchange resin include those havinganion exchange functional groups such as —N(CH₃)₂ and —NH(C₂H₄NH)_(n)H.As these anion exchange resins, commercial products such as DIAION WA10,WA20, WA21J and WA30 (products of Mitsubishi Chemical Corporation) andAMBERLITE IRA67, IRA96SB and XE583 (products of Rohm & Haas Co., Ltd.)can be used. In these commercial products, ion exchange functionalgroups are generally stabilized as those having not hydroxide ions buthalogen anions, and they are generally converted into those havinghydroxide ions such as those described above upon use of the products.

Further, gel-type anion exchange resins are classified into a crackedtype and a non-cracked type, and the non-cracked type is preferred sinceit adsorbs a less amount of bis(2-hydroxyethyl)terephthalate. Further, aporous body which is an ion exchange resin having excellent physicaldurability and a high exchange adsorption rate as compared with thegel-type anion exchange resin, i.e., a so-called MR (microporous) typecan also be used.

As for the maximum usable temperature of the cation exchange resin, itis about 120° C. for a strongly acidic styrene resin and about 100° C.for a weakly acidic methacrylic resin. As for the maximum usabletemperature of the anion exchange resin, on the other hand, it is about40 to 60° C. for a strongly basic quaternary ammonium based resin havingan —OH type exchange group, about 80° C. or lower for a strongly basicquaternary ammonium based resin having a —Cl type exchange group, andabout 100° C. or lower for a weakly basic primary, secondary or tertiaryamine (—NH₂R, —NHR₂, —NR₃) type resin. According to the abovetemperatures, after subjected to the cation exchange treatment at atemperature of, for example, 120° C. or lower, the solution compositioncan be cooled to a temperature from 40 to 60° C. so as to be subjectedto the anion exchange treatment. When bis(2-hydroxyethyl)terephthalateis precipitated due to a decrease in saturation solubility of bis(2-hydroxyethyl)terephthalate caused by a decrease in the temperature,an appropriate amount of ethylene glycol of desired temperature shouldbe added. From an economical standpoint, the anion exchange treatment isdesirably carried out by use of a primary, secondary or tertiary aminetype anion exchange resin after the cation exchange treatment is carriedout preferably at 50 to 100° C., more preferably 60 to 95° C., much morepreferably 80 to 90° C.

Illustrative examples of cations in the glycolysis reaction solutioninclude Na⁺, Ca²⁺, Mg²⁺, Zn²⁺ and Co²⁺ derived from glycolysiscatalysts, and Zn²⁺, Sb³⁺, Ge²⁺ and Ti⁴⁺ derived from polycondensationcatalysts. Meanwhile, illustrative examples of anions include PO₄ ³—derived from a stabilizer and SO₄ ²⁻ and Cl⁻ which are ions contaminatedto the polyethylene terephthalate. Since the amount of the cations ispredominantly larger than that of the anions, it is preferable to carryout the anion exchange treatment after completion of the cation exchangetreatment.

Hydrogen ions are generated by the cation exchange reaction as shown bythe following formula, and the treated solution becomes acidic.M⁺+˜SO₃H→˜SO₃M+H⁺

The generated hydrogen ions promote an ester interchange reactionbetween bis(2-hydroxyethyl)terephthalate produced by glycolysis of thepolyethylene terephthalate and diethylene glycol so as to cause2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl]terephthalate to be by-produced.

Further, when the treated solution contains a large amount of water,bis(2-hydroxyethyl)terephthalate causes hydrolysis and producesmono(2-hydroxyethyl)terephthalate.

Further, when the solution is treated at high temperatures of, forexample, 80 to 90° C., these reactions proceed faster than when thesolution is treated at room temperature.

As means for suppressing the ester interchange reaction and hydrolysisreaction of bis(2-hydroxyethyl)terephthalate, a method of neutralizingthe hydrogen ions by addition of alkali is conceivable. In this case,since cations derived from the alkali are newly brought to the system,the previously conducted cation removal treatment becomes meaninglessundesirably. Further, it is conceivable that the ion exchange treatmentsare carried out by use of a mixed layer of cations and anions. However,in consideration of regeneration of ion exchange resins arrived in breakpoint, it is preferable to carry out the cation exchange treatment andthe anion exchange treatment separately due to a large difference inamount between the cations and the anions.

As a result of intensive studies, the present inventor has found amethod of suppressing the above ester interchange reaction andhydrolysis reaction of bis(2-hydroxyethyl)terephthalate by shorteningresidence time in the cation exchange treatment. The residence time is 5to 60 minutes, preferably 5 to 50 minutes. When it is shorter than 5minutes, sufficient cation exchange treatment cannot be carried out,while when it is longer than 60 minutes, an amount ofbis(2-hydroxyethyl)terephthalate converted into2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl]terephthalate by the esterinterchange reaction becomes higher than an acceptable value (2.0%). Inaddition, the present inventor has also found that the above esterinterchange reaction and hydrolysis reaction can be suppressed bycarrying out the anion exchange treatment as soon as the cation exchangetreatment is completed. That is, when hydrogen ions produced by thecation exchange treatment are neutralized with hydroxide ions producedby the anion exchange treatment, free hydrogen ions can be reducedwithout adding additional alkali to the system, thereby suppressing theester interchange reaction and the hydrolysis reaction. The anionexchange treatment is carried out within 30 minutes, preferably within20 minutes, more preferably within 10 minutes after completion of thecation exchange treatment.

The content of ions in an ethylene glycol solution composed essentiallyof bis(2-hydroxyethyl)terephthalate after the anion exchange treatmentis preferably not higher than 2 μS/cm, more preferably not higher than 1μS/cm, in terms of electric conductivity.

EXAMPLES

Hereinafter, a more specific embodiment of the present invention will bedescribed with reference to Examples. It is needless to say that thepresent invention is not limited to the Examples only. Properties in theExamples were measured in accordance with the following methods.

(Composition of Decomposition Produced Solution)

(Separation of Components and Measurements of Amounts Thereof)

5 mg of sample was dissolved in chloroform so as to prepare about 1,000ppm of solution. In HPLC model LC-6 of Shimadzu Corporation,measurements were made at a temperature of 40° C., a flow rate of 1.0ml/min, an injection amount of 5 μl and a measurement wavelength of 240nm by use of a Silica-60 column of 4.6 mm^(ID)×250 mm^(L),dichloromethane/dioxane as a mobile phase, and a detector of aspectrophotometer for ultraviolet and visible region.

(Identification by LC/MS)

To identify a peak of HPLC, a measurement was carried out by LC/MS. Thepeak was measured and identified under the same conditions as describedabove by use of SX-102A manufactured by JEOL.

(Electric Conductivity)

This was measured continuously by means of a process conductivity meterof METTLER TOLEDO CO., LTD.

(Water Content)

This was measured by use of MK-SS type Karl Fischer Moisture Titrator ofKYOTO ELECTRONICS MANUFACTURING CO., LTD.

Example 1

(Glycolysis)

76 kg of flakes having an average size of 8 mm×8 mm and prepared bycrushing used PET bottles (comprising a polyethylene terephthalateresin) together with 10 wt % of colored PET bottles, 424 kg ofindustrial grade ethylene glycol, and 230 g of industrial grade sodiumhydroxide were charged into a 800-liter autoclave. While the mixture wasbeing stirred at a pressure of 0.13 MPa and a temperature of 215° C.,low-boiling-point materials such as water were distilled out from thetop of a distillation column disposed in the autoclave, thereby carryingout glycolysis for 110 minutes.

(Removal of Impurities)

The obtained decomposition produced solution was cooled to 180° C. so asto remove solid impurities such as caps and labels contained in theflakes by means of a 60-mesh line strainer, and the resulting solutionwas transferred to a 800-liter cooling bath. The solution was kept inthe cooling bath at 85° C. for 3 hours during which a blue pigment andother insoluble impurities were precipitated. After the precipitatedfine particles were removed by means of a 1 μm cartridge filter, thefiltrate was passed through a decolorizing column filled with activecarbon for a residence time of 115 minutes.

(Cation Exchange Treatment)

Then, the resulting filtrate was fed into a cation exchange packedcolumn (cation exchange resin: AMBERLITE IR-120B Na, product of Rohm &Haas Co., Ltd.) at 85° C., cation exchange treatment was carried out fora residence time of 12 minutes, and 10 L of the treated solution wasthen sampled so as to conduct an experiment of a shelf life.

(Anion Exchange Treatment)

Thereafter, the decomposition produced solution was passed throughconnected piping in 53 seconds, fed into an anion exchange packed column(anion exchange resin: mixture of AMBERLITE IRA96SB and AMBERLITEIR-120B Na, products of Rohm & Haas Co., Ltd.), and subjected to anionexchange treatment at 85° C. The pH of the solution was 5.2 before thecation exchange treatment, 1.76 after the cation exchange treatment, and4.9 after the anion exchange treatment. Further, the electricconductivity of the solution was 537 μS/cm before the cation exchangetreatment and 0.4 μS/cm after the anion exchange treatment.

(Purification)

The above deionized solution was cooled to 25° C. at a cooling rate of0.5° C./min, crystallized in a crystallizer for 30 minutes, and thensubjected to solid-liquid separation, thereby obtaining a wet cakecontaining 63 wt % of crude bis(2-hydroxyethyl)terephthalate. After thewet cake was molten at 100° C., it was fed into a falling-film typeevaporator so as to distill out a low-boiling-point component composedessentially of ethylene glycol at a temperature of 135° C. and apressure of 513 Pa, thereby concentrating the crudebis(2-hydroxyethyl)terephthalate. Thereafter, the resulting molten cakewas fed into a falling-film type molecular still having an internalcondenser so as to evaporate the bis(2-hydroxyethyl)terephthalate at atemperature of 208° C., a pressure of 13 Pa and a temperature of theinternal condenser of 118° C., and moltenbis(2-hydroxyethyl)terephthalate concentrated and purified by theinternal condenser was received in a receiver.

Meanwhile, residual components which flew down as thebis(2-hydroxyethyl)terephthalate was caused to evaporate along a heatedevaporation surface are extracted from a ring-shaped liquid collectionpan disposed underneath the cylindrical evaporation internal surface tothe receiver in a molten state.

(Shelf Life of Solution Subjected to Cation Exchange Treatment)

5 L of the solution after the cation exchange treatment was kept at 85°C., and a proportion of2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl]terephthalate converted frombis(2-hydroxyethyl)terephthalate in the whole solution was measured on aweight basis upon passages of 10, 20, 40, 60 and 120 minutes. As aresult, it was initially 2.8% and then increased to 3.1%, 3.5%, 4.0%,5.1% and 6.6% successively. Thereby, it was confirmed that the anionexchange treatment must be carried out as soon as the cation exchangetreatment was completed.

Example 2

Purified bis(2-hydroxyethyl)terephthalate was obtained in the samemanner as in Example 1 except that the residence time in the cationexchange treatment was changed to 30 minutes. The results are shown inTable 1.

Example 3

Purified bis(2-hydroxyethyl)terephthalate was obtained in the samemanner as in Example 1 except that the residence time in the cationexchange treatment was changed to 60 minutes. The results are shown inTable 1.

In addition, 5 wt % of water was added to the solution subjected to thecation exchange treatment, and a proportion ofmono(2-hydroxyethyl)terephthalate converted frombis(2-hydroxyethyl)terephthalate in the whole solution was measured on aweight basis upon passages of 10, 20, 40, 60 and 120 minutes. As aresult, it was 2.5% before the addition of water and then increased to3.0%, 3.6%, 4.0%, 5.7% and 9.1% successively. Thereby, it was confirmedthat mono(2-hydroxyethyl)terephthalate increased with passage of time.

Comparative Example 1

Example 1 was repeated except that the anion exchange treatment was notcarried out. As a result, distill-out ofbis(2-hydroxyethyl)terephthalate was not seen at all at the time ofmolecular distillation, and a highly viscous, light-yellow substance(melting point: 63° C.) was obtained instead. As a result of analyzingthe substance, it was an oligomer mixture containing 73.2 wt % of2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl]terephthalate. It was therebyconfirmed that both the cation exchange treatment and the anion exchangetreatment were essential.

Comparative Example 2

Purified bis(2-hydroxyethyl)terephthalate was obtained in the samemanner as in Example 1 except that the residence time in the cationexchange treatment was 2 hours and the residence time in the connectedpiping was 5 minutes. The results are shown in Table 1.

Comparative Example 3

Purified bis(2-hydroxyethyl)terephthalate was obtained in the samemanner as in Example 1 except that the solution subjected to the cationexchange treatment was reserved in a tank once and the solution was thensubjected to the anion exchange treatment after passage of 60 minutes.The results are shown in Table 1.

Comparative Example 4

Purified bis(2-hydroxyethyl)terephthalate was obtained in the samemanner as in Example 1 except that the residence time in the cationexchange treatment was 3 minutes. In this case, the electricconductivity of a solution was 537 μS/cm before the cation exchangetreatment and 12 μS/cm after the anion exchange treatment. In subsequentmolecular distillation, distill-out of the purifiedbis(2-hydroxyethyl)terephthalate was little, and distill-out of oligomerwas increased instead. It was thereby confirmed that when the cationexchange treatment is insufficient, residual cations serving as acatalyst promote conversion of bis(2-hydroxyethyl)terephthalate into anoligomer.

Table 1 shows proportions of2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl]terephthalate converted frombis(2-hydroxyethyl)terephthalate in the whole solutions in Examples 1 to3 and Comparative Examples 2 and 3. Optical densities in Table 1 areused to evaluate the quality of bis(2-hydroxyethyl)terephthalate and arevalues considered to be proportional to the content of colorant. Morespecifically, they are values resulting from measuring the absorbance of10-wt % methanol solution of bis(2-hydroxyethyl)terephthalate at a celllength of 10 mm and a wavelength of 380 nm.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 C. Ex. 2 C. Ex. 3 Time from Start of Cation12.9 30.9 60.9 125 180 Exchange Treatment to Start of Anion ExchangeTreatment (minute) 2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl]terephthalate (wt %) In Solid Content Before CationExchange 1.31 1.31 1.32 1.30 1.31 In Solid Content After Cation Exchange2.63 2.69 3.80 11.41 11.37 In Cake Resulting from Crystallization 1.721.98 2.39 6.68 16.66 and Solid-Liquid Separation In Purified bis(2- 1.411.60 1.93 3.92 9.78 hydroxyethyl)terephthalate Optical Density ofPurified 0.000 0.001 0.002 0.077 0.154 bis(2-hydroxyethyl)terephthalateEx.: Example, C. Ex.: Comparative Example

EFFECTS OF THE INVENTION

As described above, according to the method of the present invention fordeionizing a decomposition produced solution resulting fromdecomposition of a polyester by ethylene glycol, the ester interchangereaction and hydrolysis reaction of the decomposition product along withcation removing treatment can be suppressed. Consequently, a method fordeionizing the above decomposition produced solution with smallreductions in yield and purity can be provided.

1. A method for deionizing a decomposition produced solution having awater content of not higher than 0.5 wt % resulting from decompositionof a polyester by ethylene glycol, with the weight ratio between thepolyester and ethylene glycol of 1:9 to 3:7, wherein duringdecomposition water is distilled out of the solution and evaporatedethylene glycol is returned into the solution, the method furthercomprising: bringing the solution containing a decomposition product,ethylene glycol and cations and anions as impurities into contact with acation exchanger at a temperature of not higher than 100° C. for aresidence time of 5 to 60 minutes and then with an anion exchangerwithin 10 minutes after the contact with the cation exchanger so as toreduce contents of the cations and anions.
 2. The method of claim 1,wherein the anion exchanger consists of a mixed layer of the cationexchanger and the anion exchanger.
 3. The method of claim 2, wherein adecomposition produced solution prepared by decomposing a polyester byethylene glycol while water is distilled out is used.
 4. The method ofclaim 2, wherein solid impurities each of which is not smaller than 1 μmare removed from the decomposition produced solution before the solutionis brought into contact with the cation exchanger and the anionexchanger.
 5. The method of claim 2, wherein the polyester ispolyethylene terephthalate.
 6. The method of claim 2, wherein theelectric conductivity of decomposition produced solution after the anionexchange treatment is not higher than 1 μS/cm.
 7. The method of claim 1,wherein solid impurities each of which is not smaller than 1 μm areremoved from the decomposition produced solution before the solution isbrought into contact with the cation exchanger and the anion exchanger.8. The method of claim 7, wherein the polyester is polyethyleneterephthalate.
 9. The method of claim 7, wherein the electricconductivity of decomposition produced solution after the anion exchangetreatment is not higher than 1 μS/cm.
 10. The method of claim 1, whereinthe polyester is polyethylene terephthalate.
 11. The method of claim 10,wherein the electric conductivity of decomposition produced solutionafter the anion exchange treatment is not higher than 1 μS/cm.
 12. Themethod of claim 1, wherein the electric conductivity of decompositionproduced solution after the anion exchange treatment is not higher than1 μS/cm.
 13. The method of claim 1, wherein solid impurities each ofwhich is not smaller than 1 μm are removed from the decompositionproduced solution before the solution is brought into contact with thecation exchanger and the anion exchanger.
 14. The method of claim 1,wherein the polyester is polyethylene terephthalate.
 15. The method ofclaim 1, wherein the electric conductivity of decomposition producedsolution after the anion exchange treatment is not higher than 1 μS/cm.